The invention relates to a process and an installation for the dry fractionation of edible oils, fats and related products.
Dry fractionation of edible oils and fats is practiced industrially on a large scale and a number of different processes is used for this purpose. Commonly, the oil to be fractionated is fed to a crystallizer where it is cooled so that fat crystals are formed. These crystallizers can be large vessels provided with cooling coils and an agitator to assist heat transfer and prevent the crystals from settling, but other types of crystallizers such as scraped surface heat exchangers and other dynamic crystallizers, are also being used.
The resulting crystal slurry is then subjected to a filtration process to separate these crystals from their mother liquor. The filter cake is referred to as the stearin fraction and the filtrate, the mother liquor, is referred to as the olein fraction. Various filtration systems are used, ranging from a simple band or drum filter to high pressure, fully automated membrane filter presses.
This process works well for oils and fats that form separate, individual crystals on cooling such as palm oil and its fractions, anhydrous milkfat, edible tallow, cottonseed oil, hydrogenated vegetable oils etc.
However, certain oils and fats, notably lard, palm kernel oil and coconut oil tend to form a network of fine crystals rather than separate crystals on cooling, so that a solid mass instead of a pumpable slurry results.
Therefore, other types of fractionation processes are used for such materials. Solvent fractionation is one of these processes, but this is an expensive process. Because of the inflammable nature of the solvent, the plant has to be explosion proof and this greatly increases the investment required. In addition, the evaporation and rectification of the solvent are energy intensive processes.
Therefore, the most commonly used fractionation process for oils like palm kernel oil comprises the solidification of the oil in trays or pans to form blocks, wrapping these blocks individually in a filter cloth and loading the blocks into a hydraulic press to squeeze the olein from between the crystals. The investment required for this process may be fairly low but wrapping the blocks and the subsequent unwrapping of the filter cake make it very labour intensive.
Accordingly, attempts have been made to develop processes that require less labour and that use for instance an automated membrane filter press. U.S. Pat. No. 4,795,569 to Higuchi et al. describes such a process in which the oil is introduced into a filter chamber and allowed to crystallize inside that chamber by circulating a coolant such as water through the space between the membrane and a filter frame.
However, this process requires the filter cloth to be sealed first with a coagula of the material to be treated. This makes it a lengthy process that makes inefficient use of the expensive membrane press.
Accordingly, an improvement has been described in U.S. Pat. No. 5,045,243 to Kuwabara et al. in which the oil or fat to be fractionated is first of all solidified in trays to form solid blocks which are then crushed to yield a pumpable paste that is then introduced into a membrane press to separate this paste into an olein fraction and a stearin fraction. The solidification process is commonly carried out in cooling tunnels. However, these have the disadvantage that the oil is exposed to the air while being cooled and that it is virtually impossible to control the rate of cooling inside the individual trays.
Therefore, an apparatus has been described in European Patent Application 1.028.159 by Yoneda et al. that permits stationary crystallization. In this apparatus, the oil or fat to be fractionated is not solidified into a solid block, but the crystallization process is halted when the partially crystallized mass is still sufficiently fluid to be pumped into the membrane filter press. However, this means that the material to be fractionated has to be diluted with olein before being cooled. This recycling of olein greatly reduces the actual capacity of the apparatus.